1. Field of the Invention
The present invention relates to a seismic survey system. More particularly, the present invention relates to a wireless land seismic survey system.
2. Art Background
Land seismic survey operations typically require placing hundreds to thousands of geophones at locations about the area to be surveyed. When a seismic source is generated, either as an impulse caused by dynamite or a sweep caused by a Vibroseis truck, the seismic reflections are detected by the geophones. The measurement data generated by all the geophones is then transmitted to a central recording system.
The amount of data transmitted to the central recording system may be considerable. For example a 20-second Vibroseis sweep can generate on the order of 250,000 bits of data. When there are 1,000 geophone channels in use, this translates to 250,000,000 bits of data every 20 seconds or an effective data rate of 12.5 megabits per second.
Increasing the number of geophone channels increases the amount of data to be transferred to the central recording system. Many current seismic survey projects have more than 1,000 geophones active at any one time and the requirements for more channels is increasing. In a few years time it is expected that channel counts as high as 10,000 will not be uncommon.
Most seismic systems utilize cables to transfer the seismic data from the geophones to the central recording system. The cables are typically laid out in parallel with a "cross-connect" cable to collect the data from the several parallel cables. With very high channel counts there can be "cross-connect" cables cross-connecting lower level "cross-connect" cables. When there are thousands of channels for a seismic survey project the amount of cable that has to be placed on the ground is considerable. For very large projects this can amount to 50 miles or more of cable
50 miles of cable weighs on the order of 40 tons. Consequently laying out this amount of cable is expensive and difficult. It is equally expensive and difficult to pick up upon completion of the seismic survey project.
In addition to the physical effort required to lay out the cable there are other problems with cable operations. In some areas rodents and other animals chew through the cables and the ensuing breaks have to be located and repaired. Lightning strikes can cause sever cable damage which can cause problems requiring several days to isolate and repair.
Some cable problems are obvious and are simple to identify and fix. A cable cut in two is such an example. Sometimes the problems are intermittent. These problems can take days to locate and repair. Each day a seismic survey crew is occupied trying to identify and repair cable problems represents an economic loss to the project.
Some seismic survey systems use radio links to collect geophone data. With radios in lieu of cables there is no cable layout required and obstacles to cable operations such as swamps are not a problem. However, as currently implemented, radio-based systems are very constrained as to the number of geophones or channels they can support. Typically there is sufficient bandwidth available to support only a few hundred channels.
What is needed is a radio-based system for eliminating the problems associated with cable-based systems and which is not constrained as to the number of channels that can be supported.